CFG_CPU_FREQ cancels itself out in the original equation: ((CFG_CPU_FREQ / CFG_SYSTICK_FREQ) / (CFG_CPU_FREQ / 1000)) In addition, comment is now fixed - 1ms is used for STM32, 2ms was used for other chips: https://github.com/opentx/opentx/blob/e94f6c0ccdc1ecd800f2ef27a28120878bc9ec41/radio/src/thirdparty/CoOS/OsConfig.h#L88-L92